Traditionally, various core cutters of this type have been proposed. For example, core cutters are entirely formed of the same material, to be specific, high-speed steel. The core cutter formed of the same material is provided with a spiral groove on a cylindrical outer peripheral surface thereof. Through the spiral groove, chips resulting from cutting operation are discharged. A portion of the core cutter except the spiral groove is a substantial thickness of a base of the core cutter.
In the core cutter for drilling holes in metal, resin material and other materials, since the chips being generated continuously have a large width, it is desirable to minimize the width of the chips in order to discharge the chips efficiently outside the holes.
To this end, each cutting portion at a tip end portion of the core cutter is provided with a plurality of cutting blades defining a width of the chips, and is configured to discharge the chips with a width equal to the width of the cutting blades of each cutting portion. For example, each cutting portion has a first cutting blade located radially inward and a second cutting blade located radially outward.
In the core cutter thus constructed, the first cutting blade is formed radially inward relative to the discharge groove and the second cutting blade is formed radially outward relative to the discharge groove.
In a core cutter of another configuration, each cutting portion has first to third cutting blades in this order from radially inward. In the core cutter thus constructed, each cutting blade has a gallet for discharging the chips which is formed by a step portion. And, the chips cut by each cutting blade of each cutting portion are discharged through each gallet to outside from a discharge groove continuous with a base end side of each gallet (see Japanese Laid-Open Patent Application Publication 58-59712).
However, in the former cutter, the thickness (radial wall thickness) of the base is determined by a radial thickness of the first cutting blade (or first and second cutting blades) of the cutting portion. Therefore, in the cutter thus constructed, the wall thickness (thickness) of the base decreases depending on the structure of the cutting portion (thickness of the cutting portion), and rigidity of the base becomes lower than a desired value. In this case, the cutter is unable to conduct heavy cutting or cut the material with high cutting resistance.
In the latter core cutter, since the cutting blades are provided with different gallets, time and complicated process are required to form the gallets with intricate configurations.